1. Field of the Invention
The present invention relates to a monitor, and more particularly to an electric power cut-off detection unit for a monitor capable of preventing damage to the monitor by promptly judging an electric power cut-off.
2. Description of the Prior Art
FIG. 1 is a brief diagram for showing a conventional electric power cut-off detection unit for a monitor. As shown in FIG. 1, the conventional electric power cut-off detection unit has a full-wave rectifier 1. An alternate electric power AC, for example 110 V or 220 V, is applied to full-wave rectifier 1. Full-wave rectifier 1 full-wave rectifies alternative electric power AC and outputs a certain direct voltage DC1, for example 5 V or 12 V. Full-wave rectifier 1 is connected with a first voltage generator 2. Direct voltage DC1 of full-wave rectifier 1 is inputted to first voltage generator 2. First voltage generator 2 rectifies direct voltage DC1 and generates a direct voltage SDCC1 more stabilized than direct voltage DC1. That is, direct voltage DC1 of full-wave rectifier 1 is supplied to a regulator REG1 of first voltage generator 2. Regulator REG1 rectifies direct voltage DC1 and generates a direct voltage SDC1 more stabilized than direct voltage DC1. Regulator REG1 is connected in series with a capacitor C1. Direct voltage SDC1 of regulator REG1 is charged into capacitor C1. According to direct voltage SDC1 of regulator REG1 (direct voltage SDC1 is a voltage with ripple components), capacitor C1 repeatedly charges and discharges its charged direct voltage SDC1. Therefore, first voltage generator 2 generates direct voltage SDCC1 more stabilized than direct voltage SDC1 of regulator REG1.
First voltage generator 2 is electrically connected with a second voltage generator 3. Direct voltage SDCC1 of first voltage generator 2 is applied to second voltage generator 3. Second voltage generator 3 generates a direct voltage SDCC2 lower than direct voltage SDCC1. Direct voltage SDCC1 of first voltage generator 2 is supplied to a regulator REG2 of second voltage generator 3. Regulator REG2 lowers and regulates direct voltage SDCC1 and outputs a direct voltage SDC2 lower than direct voltage SDCC1. Regulator REG2 is electrically connected in series with a capacitor C2. Direct voltage SDC2 of regulator REG2 is charged into capacitor C2. According to direct voltage SDC2 of regulator REG2 (direct voltage SDC2 is a voltage with ripples), capacitor C2 repeatedly charges and discharges its charged direct voltage SDC2. Therefore, second voltage generator 3 generates direct voltage SDCC2 more stabilized than direct voltage SDC2 of regulator REG2. Second voltage generator 3 is electrically connected with a power cut-off detector 5. Direct voltage SDCC2 of second voltage generator 3 is supplied to power cut-off detector 5. Power cut-off detector 5 detects a variation of direct votage SDCC2, for example a variation from 5 V to 0 V, in accordance with a cut-off of alternate electric power AC and outputs a detection signal CUT5. That is, as shown in FIG. 1, direct voltage (SDCC2) of second voltage generator 3 is divided by resistors R1 and R2 of power cut-off detector 5. A voltage VR2 across resistor R2 is applied to the inverting terminal of a comparator COM. Further, direct voltage SDCC2 of second voltage generator 3 is charged into a capacitor C3 through a resistor R3 and a diode D1 of power cut-off detector 5. A voltage VC3 charged in capacitor C3 is applied to the non-inverting terminal of comparator COM. Comparator COM compares voltage VR2 of resistor R2 with voltage VC3 of capacitor C3 and outputs detection signal CUT5. Detection signal CUT5 is applied to a microprocessor 4 built in a monitor. On a power cut-off of alternate electric power AC, comparator COM outputs a high level, for example 5 V, as detection signal CUT5 to microprocessor 4 since voltage VR2 of resistor R2 is lowered to a low level, for example 0 V, and voltage VC3 of capacitor C3 is maintained to a high level, for example direct voltage SDCC2. According to an input of detection signal CUT5 of the high level, microprocessor 4 stores data processed at present, for example contrast data, picture size data and chip data in a deflection coil, for preventing data loss inputted till now by user or data breakage in chip of the deflection coil. When alternate electric power AC is normally supplied, comparator COM outputs a voltage of a low level as detection signal CUT5 to microprocessor 4 since voltage VR2 of resistor R2 and voltage VC3 of capacitor C3 are respectively maintained to a high level, for example direct voltage SDCC2. According to an input of the voltage of the low level, microprocessor 4 continues to perform its present operations. U.S. Pat. No. 4,733,158 describes a power supply, in which the power supply has a function of controlling its output voltage according to the variations of input voltages.
There exists a drawback in that such a conventional electric power cut-off detection unit has its complicated construction since a comparator and a diode should be employed for detecting a cut-off of electric power. Further, there exists a drawback in that a time period is short for a microprocessor to take safety measures on a power cut-off since the power cut-off is detected with a voltage outputted from a second regulator.